The present embodiments relate to wireless communications systems and are more particularly directed to improving signal-to-noise ratio (“SNR”) by processing signal paths that are relatively close together in time.
Wireless communications have become very prevalent in business, personal, and other applications, and as a result the technology for such communications continues to advance in various areas. One such advancement includes the use of spread spectrum communications, including that of code division multiple access (“CDMA”). CDMA systems are characterized by simultaneous transmission of different data signals over a common channel by assigning each signal a unique code. This unique code is matched with a code of a selected user station within the cell to determine the proper recipient of a data signal.
Wireless networks are now being designed for a variety of devices that are typically within fairly close distances of one another, such as in the range of 10 meters or less. In the current state of the art, such a network is sometimes referred to as a personal area network (“PAN”) and it may include, by way of example, a keyboard and a printer, each of which communicates in a wireless manner with a mutual computer that is also part of the PAN. Another type of commonly used wireless network is a cellular telephone system. In such communications, a user station (e.g., a hand held cellular phone) communicates with a base station, where typically the base station corresponds to a “cell.” In any event, for both types of systems, other devices also may be implemented, and the term network is used in this document to describe a system consisting of an organized group of any of various types of intercommunicating devices.
CDMA continues to advance along with corresponding standards that have brought forth a next generation wideband CDMA (“WCDMA”). WCDMA includes alternative methods of data transfer, one being time division duplex (“TDD”) and another being frequency division duplex (“FDD”). The present embodiments are preferably incorporated in FDD but alternatively also may apply to TDD; thus, both approaches are further introduced here. TDD data are transmitted in one of various different forms, such as quadrature phase shift keyed (“QPSK”) symbols or other higher-ordered modulation schemes such as quadrature amplitude modulation (“QAM”) or 8 phase shift keying (“PSK”). In any event, the symbols are transmitted in data packets of a predetermined duration or time slot. Within a data frame having 15 of these slots, bi-directional communications are permitted, that is, one or more of the slots may correspond to communications from a base station to a user station while other slots in the same frame may correspond to communications from a user station to a base station. Further, the spreading factor used for TDD is relatively small, whereas FDD may use either a large or small spreading factor. FDD data are comparable in many respects to TDD including the use of 15-slot frames, although FDD permits a different frequency band for uplink communications (i.e., user to base station) versus downlink communications (i.e., base to user station), whereas TDD uses a single frequency in both directions.
Due to various factors including the fact that CDMA communications are along a wireless medium, an originally transmitted communication from a transmitter to a receiver may arrive at the receiver at multiple and different times. Each different arriving signal that is based on the same original communication is said to have a diversity with respect to other arriving signals originating from the same transmitted communication. Further, various diversity types may occur in CDMA communications, and the CDMA art strives to ultimately receive and identify the originally transmitted data by exploiting the effects on each signal that are caused by the one or more diversities affecting the signal. One type of CDMA diversity may occur in many indoor or pedestrian environments because a transmitted signal from a station is reflected by objects that it contacts. For example, in an outdoor environment these objects may be the ground, mountains, and buildings, while in an indoor environment these objects may be walls and furniture. As a result, a same single transmitted communication may arrive at a receiving station at numerous different times, and assuming that each such arrival is sufficiently separated in time, then each different arriving signal is said to travel along a different channel and arrive as a different “path.” These multiple signals are referred to in the art as multiple paths or multipaths. Several multipaths may eventually arrive at the receiving station and the channel traveled by each may cause each path to have a different phase, amplitude, and signal-to-noise ratio (“SNR”). Accordingly, for one communication from one station to another station, each multipath is originally a replica of the same originally transmitted data, and each path is said to have time diversity relative to other multipath(s) due to the difference in arrival time which causes different (uncorrelated) fading/noise characteristics for each multipath.
According to the prior art, although multipaths carry the same user data to the receiver, they are separately recognized by the receiver based on the timing of arrival of each multipath to improve performance. More particularly, CDMA communications are modulated using a spreading code which consists of a series of binary pulses, and this code runs at a higher rate than the symbol data rate and determines the actual transmission bandwidth. In the current industry, each piece of CDMA signal transmitted according to this code is said to be a “chip,” where each chip corresponds to an element in the CDMA code. Thus, the chip frequency defines the rate of the CDMA code. Given the transmission of the CDMA signal using chips, then according to the prior art multipaths separated in time by at least one chip are distinguishable at the receiver by assigning a different resource to process each corresponding multipath. Such an operation for large spreading factor FDD communications is often performed by assigning each identified path to a corresponding finger of a rake receiver, where the rake receiver performs what is referred to as a maximal ratio combining (“MRC”) operation which combines the various paths taking into account the respective delays of those paths and in view of the low auto-correlations of CDMA codes. Accordingly, given that numerous multipaths may arrive at a receiving user station, the prior art endeavors to select certain of those multipaths and then to perform various processing on those paths in an effort to combine the signals to remove the effects of the diversity and to better recover the originally-transmitted data represented by those signals.
While the preceding prior art operations have proven useful in identifying multipaths for further processing, the present inventors have observed that such an approach yields an SNR that may be improved further. Specifically, the present inventors have observed a drawback in that the prior art approach as described above limits itself to process paths that are at least one chip apart. As described below with respect to the preferred embodiments, additional gains may be achieved in SNR by processing multipaths under an analysis that considers paths that are within one chip of another, that is, they fall within a sub-chip boundary in time.